1. Field of the Invention
The present invention relates to a method for recovering ethylene from vent gas from a plant for production of ethylene oxide. More specifically, the present invention relates to a method for separating ethylene from saturated hydrocarbon such as methane, ethane and oxygen contained in the vent gas, by which the ethylene can be recovered efficiently.
2. Description of the Prior Art
In the plants for producing ethylene oxide by oxidation of ethylene, the process gas obtained after recovering ethylene oxide from the effluent reaction gas contains a large amount of unreacted ethylene. Therefore, the process gas is recirculated back into the reaction system in order to enhance the reaction efficiency of ethylene.
The process gas also contains, besides the unreacted ethylene, saturated hydrocarbons such as methane and ethane, inert gases such as carbon dioxide, argon and nitrogen and oxygen. As a result of the repeated circulation of the gas into the reaction system, the inert gases, such as nitrogen and argon, are concentrated, and a part of the concentrates are continuously withdrawn out of the reaction system to be utilized as fuel gas.
The vent gas, however, usually contains about 30% ethylene, and it is uneconomical to use the gas only as fuel gas. Therefore, in order to re-use the gas as raw material gas, various methods for recovering ethylene from the vent gas have been proposed wherein the vent gas is treated with silica gel or active carbon to adsorb the ethylene and then desorbing and recovering the ethylene therefrom (see, for example, Japanese Patent Application Laid-open Nos. 109117/1983, 174732/1985 and 116721/1988).
However, these methods have the following disadvantages:
(i) As even methane, contained in the vent gas in a larger amount than ethylene, is adsorbed as well as ethylene, the amount of gas recovered by means of desorption is increased. Generally, in the re-circulation to the ethylene oxide-production plant, the recovered gas must be repressurized to the reaction pressure (normally 20 kg/cm.sup.2). From the industrial viewpoint, it is greatly uneconomical to pressurize the gas accompanying methane which is inert for the oxidation reaction.
(ii) The complete separation and removal of inert gases (e.g. argon, nitrogen, carbonic acid gas, etc.) contained in the vent gas surely causes loss of ethylene into the exhaust gas; whereas, the improvement of recovery efficiency of ethylene results in an increase in concentration of the inert gases such as argon in the recovered gas; which is not desirable.
Accordingly, it is difficult for such conventional methods to avoid the loss of ethylene and give the recovered gas containing ethylene in a high concentration efficiently.